The present invention relates to the field of hydrodynamic bearing assemblies, and especially to such assemblies adapted to have good stiffness and long useful life.
Many motors, spindles and the like are based on bearing cartridges comprising a shaft and sleeve and bearings supporting these two elements for relative rotation. For example, a shaft may be mounted by means of two ball bearings to a sleeve rotating around the shaft. One of the bearings is typically located at each end of the shaft/sleeve combination. These bearings allow for rotational movement between the shaft and the hub while maintaining accurate alignment of the sleeve to the shaft. The bearings themselves are normally lubricated by grease or oil.
The conventional bearing system described above is prone, however, to several shortcomings. First is the problem of vibration generated by the balls rolling on the raceways. Ball bearings in such cartridges frequently run under conditions that result in physical contact between raceways and balls; this occurs in spite of the lubrication layer provided by the bearing oil or grease. Hence, bearing balls running on the generally even and smooth, but microscopically uneven and rough raceways, transmit this surface structure as well as their imperfections in sphericity in the form of vibration to the rotating element. This vibration results in misalignment between whatever device is supported for rotation and the surrounding environment. This source of vibration limits therefore the accuracy and the overall performance of the system incorporating the cartridge.
Another problem is related to damage caused by shocks and rough handling. Shocks create relative acceleration between stationary and rotating parts of a system which in turn shows up as a force across the bearing system. Since the contact surfaces in ball bearings are very small, the resulting contact pressures may exceed the yield strength of the bearing material and leave permanent deformation and damage on raceways and balls, which would also result in tilt, wobble, or unbalanced operation of the bearing.
Moreover, mechanical bearings are not always scalable to smaller dimensions. This is a significant drawback since the tendency in the high technology industry has been to continually shrink the physical dimensions.
As an alternative to conventional ball bearing spindle systems, researchers have concentrated much of their efforts on developing a hydrodynamic bearing. In these types of systems, lubricating fluidxe2x80x94either gas or liquidxe2x80x94functions as the actual bearing surface between a stationary base or housing and the rotating spindle or rotating hub and the stationary surrounding portion of the motor. For example, liquid lubricants comprising oil, more complex ferro-magnetic fluids, or even air have been utilized for use in hydrodynamic bearing systems. Such bearings scale well to small sizes without being prone to many of the defects of ball bearings outlined above. Because of the lack of metal-to-metal contact, the bearing has a long life. Because of the stiffness of the bearing, it is highly stable and useful as a reference in devices such as optical encoders and the like.
However, it is apparent that a difficulty with such a hydrodynamic bearing design is their sensitivity both to machining tolerances and the temperature ranges across which they are utilized. Both of these issues are critical in hydrodynamic bearings, because the very narrow gaps between the rotating and stationary parts must be maintained so that the fluid is effective in lubricating the bearing surfaces. Further, the tolerances between the surfaces of the bearing must be very fine so that no tilting or misalignment between the two parts occurs. In other words, it is important to have a very stiff bearing which does not allow for any tilting of the rotating part relative to the stationary part. A further difficulty with prior art designs is that frequently voids or gas bubbles occur in the bearing area, thereby reducing the effective bearing surface and the related load capacity.
Thus it is clear that a number of considerations must be balanced in designing an effective hydrodynamic bearing cartridge, regardless of the area in which it will eventually be utilized.
It is therefore a primary objective of the present invention to provide a hydrodynamic bearing which is simple in design, and highly adaptable and scalable for use in many different environments. It is a further objective of the invention to provide a hydrodynamic bearing having a reliable, repeatable design so that the bearing has the necessary stiffness to be used in applications which have no tolerance for tilt, wobble, or other inaccuracies.
It is a further and related objective of the present invention to provide a hydrodynamic bearing in which the fluid circulation is controlled and directed so that the wear and tear on the two prior surfaces defining the bearing is minimized.
Another related objective of the present invention is to provide for fluid circulation within the hydrodynamic bearing such that the possibility of voids within the lubricant is minimized.
A related objective of the invention is to provide a hydrodynamic bearing design having optimized boundary conditions between the various sections of the bearings to optimize fluid flow and diminish sensitivity to temperature and machining tolerances, thereby providing a greater consistency in the dynamic performance of the invention.
These and other objectives are achieved by providing a hydrodynamic bearing having a shaft relatively rotatable with respect to a surrounding sleeve and having a thrust plate on one end thereof rotating in a recess of the sleeve. The shaft is preferably interrupted by a equi-pressure groove accessing a central reservoir in the shaft and having journal bearings defined by herringbone patterns above and below the groove to stabilize and provide stiffness to the cartridge. The stiffness of the cartridge is further enhanced by a thrust plate carried at one end of the shaft and rotating in a recess of the sleeve and being used to define thrust bearings on either surface thereof. In a typical embodiment, chevron patterns may be coined or etched on both surfaces of the thrust plate so that appropriate pressure patterns can be set up between the thrust plate surface and either a shoulder of the sleeve or a facing counterplate. Alternatively, a counterplate may be provided in which the chevron pattern is stamped thereon, and may in a preferred embodiment even extend beyond the edges of the thrust plate and the recess in which it rotates so that disturbances to the pressure patterns are minimized.